This section provides background information related to the present disclosure which is not necessarily prior art.
Cooling systems have applicability in a number of different applications where fluid is to be cooled. They are used in cooling gas, such as air, and liquids, such as water. Two common examples are building HVAC (heating, ventilation, air conditioning) systems that are used for “comfort cooling,” that is, to cool spaces where people are present such as offices, and data center climate control systems.
A data center is a room containing a collection of electronic equipment, such as computer servers. Data centers and the equipment contained therein typically have optimal environmental operating conditions, temperature and humidity in particular. Cooling systems used for data centers typically include climate control systems, usually implemented as part the control for the cooling system, to maintain the proper temperature and humidity in the data center.
An example of a prior art cooling system is the DSE™ cooling system product line available from Liebert Corporation of Columbus, Ohio. FIG. 3 is a basic schematic showing an example configuration of a DSE™ cooling system 300. Cooling system 300 includes a direct expansion (“DX”) cooling circuit 302 having an evaporator 304, expansion valve 306 (which may preferably be an electronic expansion valve but may also be a thermostatic expansion valve), condenser 308 and compressor 310 arranged in a DX refrigeration circuit. Cooling circuit 302 also includes a pump 312, solenoid valve 314, check valves 316, 318 and 320, and receiver/surge tank 324. An outlet 328 of condenser 308 is coupled to an inlet 326 of receiver/surge tank 324. An outlet 330 of receiver/surge tank 324 is coupled to inlet 334 of pump 312 and to inlet 336 of check valve 316. An outlet 344 of pump 312 is coupled to an inlet 346 of solenoid valve 314. An outlet 348 of solenoid valve 314 is coupled to an inlet 350 of electronic expansion valve 306. An outlet 352 of check valve 316 is also coupled to the inlet 350 of electronic expansion valve 306. An outlet 354 of electronic expansion valve 306 is coupled to a refrigerant inlet 356 of evaporator 304. A refrigerant outlet 358 of evaporator 304 is coupled to an inlet 360 of compressor 310 and to an inlet 362 of check valve 318. An outlet 364 of compressor 310 is coupled to an inlet 366 of check valve 320 and an outlet 368 of check valve 320 is coupled to an inlet 370 of condenser 308 as is an outlet 372 of check valve 318.
Cooling system 300 also includes a controller 374 coupled to controlled components of cooling system 300, such as electronic expansion valve 306, compressor 310, pump 312, solenoid valve 314, condenser fan 378, and evaporator air moving unit 332. Controller 374 is illustratively programmed with appropriate software that implements the control of cooling system 300. Controller 374 may include, or be coupled to, a user interface 376. Controller 374 may illustratively be an iCOM® control system available from Liebert Corporation of Columbus, Ohio programmed with software implementing the control of cooling system 300 including the additional functions described below. In this regard, controller 374 may be programmed with software implementing the control described in U.S. Ser. No. 13/446,310 for “Vapor Compression Cooling System with Improved Energy Efficiency Through Economization” filed Apr. 13, 2012. The entire of disclosures of U.S. Ser. No. 13/446,310 is incorporated herein by reference.
Pump 312 may illustratively be a variable speed pump but alternatively may be a fixed speed pump. Condenser fan 378 may illustratively be a variable speed fan but alternatively may be a fixed speed fan. It should be understood solenoid valve 314 could be types of controlled valves other than solenoid valves, such as a motorized ball valve or variable flow valve.
It should be understood that pump 312, solenoid valve 314 and check valve 316 are basic elements of an optional unit in the DSE™ product line known as the ECONOPHASE™ unit, identified in phantom in FIG. 3 with reference number 380, having an inlet 382 at a junction of inlet 334 of pump 312 and inlet 336 of check valve 316 and an outlet 384 at a junction of outlet 348 of solenoid valve 314 and outlet 352 of check valve 316. It should thus be understood that cooling system 300 can be configured without ECONOPHASE™ unit 380 with the outlet 330 of receiver/surge tank 324 coupled to the inlet 350 of electronic expansion valve 306.
In the DSE™ product line, condenser 308 is a micro-channel condenser. That is, condenser 308 has one or more micro-channel cooling coils referred to herein as micro-channel cooling coil 309. Evaporator 304 is a fin-and-tube evaporator. That is, evaporator has one or more fin-and-tube cooling coils referred to herein as fin-and-tube cooling coil 305. As is known in the art, a typical fin-and-tube cooling coil has rows of tubes (usually copper) that pass through sheets of formed fins (usually aluminum). The rows of tubes may be one or more tubes having a serpentine configuration that snakes back and forth. Also as known in the art, a typical micro-channel cooling coil has a series of parallel flat micro-channel tubes extending between inlet and outlet manifolds with fins extending between the adjacent micro-channel tubes. Each micro-channel tube has a series of micro-channels therein extending the length of the tube. A micro-channel is typically defined as a channel (flow passage) with a hydraulic diameter in the range of 10 to 1000 micrometers.
Micro channel cooling coils offer many benefits compared to tube and fin cooling coils. Low internal refrigerant volume and smaller footprint are among them. The low internal refrigerant volume means that the micro-channel cooling coil holds much less refrigerant charge than an equivalent sized tube-and fin cooling coil. While this is beneficial from a cost standpoint, it causes an issue in the operation of the system. The low amount of refrigerant causes the system to be very sensitive to the total amount of system refrigerant charge. Small amounts of charge difference can equate to significant changes in sub-cooling due to the amount of liquid refrigerant in the condenser and the low volume of refrigerant relative to the coil face area. Also, if the volume of the evaporator is large relative to the volume of the condenser, this creates an issue with migration of charge and how the system handles this charge during a change in ambient temperatures of the evaporator and/or the condenser. For example, when the ratio of the evaporator volume (the volume of refrigerant charge that the fin-and tube cooling coil of evaporator holds) to condenser volume (the volume of refrigerant charge that the micro-channel cooling coil of the condenser holds) is greater than 2.5, there may be issues with charging of the system. If the system is charged with refrigerant when cold outside (at condenser) and warm inside (at evaporator) the system will be overcharged when run with an opposite swing in temperatures (cold indoor and warm outdoor). In this scenario, refrigerant migration will result in high discharge pressures and very likely trip the high pressure cut-out safety device. In the opposite case, if the unit were charged when cold inside (at evaporator) and warm outside (at condenser), the unit will lose its sub-cooling when run at the opposite conditions (warm indoor and cold outdoor) such that capacity and efficiency will be significantly reduced.
To address the above discussed refrigeration migration charge issue, a large receiver/surge tank 324 has been added on the discharge side of condenser 308 to allow for migration of refrigerant. This receiver/surge tank 324 is required due to the relative difference between the volume of condenser 308 and the volume of evaporator 304 as the volume of condenser 308 is small relative to the volume of evaporator 304. It was determined that when the ratio of the volume of evaporator 304 to condenser 308 is greater than 2.5, cooling system 300 system may not be able to function properly throughout the required range of operation (outdoor air temperature between −30° F. and 105° F. and return air temperature to the evaporator between 68° F. and 105° F.). Receiver/surge tank 324 was thus added at the discharge of condenser 308 to hold additional volume of refrigerant. However, when a receiver/surge tank is added to the system, sub-cooling of refrigerant out of the condenser is lost with a corresponding loss of efficiency and capacity.